A common way to decompose logs into boards and planks is that in a first stage in a so-called stock-taking saw, cut a rectangular block by cutting off the sides, called back sides of the log. The block that remains to divide in a second step in a second-coming so-called division saw has a rectangular cross section, but is usually more or less curved in the longitudinal direction, depending on the log curve. Curve sawing is therefore a usual manner by sawing recut to decompose the curved block and thus gain advantages as to quality and increased yield, as a result of following the grain of the wood to a greater extent compared to straight sawing. It may be added that curve sawed boards later straightens out when drying.
Curve sawing comprises feeding the curved block past the division saw in a curved path that follows the log's curvature. In some cases, a different curve radii when curve sawing along a log. This is due to a rootstock often is crooked in the root end, but may be straightened in the top end. The division saw may be a single circular saw blade (single blade) or double circular saw blades (double blades or twin blades).
When sawing recut with double blades, or twin blades, these often are conflicting and displaced in the feed direction to permit vertical overlap between the circular saws and a certain safety margin to prevent the saw blades from coming into contact with each other. In some cases the overlying and the underlying saw blade may be parallel to each other in a vertical plane, i.e. lack displacement in the feeding direction in relation to each other. In such case, the rotation of the saw blades may be synchronised with each other so that the saw teeth on each respective the saw blade engage in the gap between the serrations on the opposite saw blade, much like the teethes of a pair of inter acting gears, but without touching each other.
A problem occurring at such tight curve sawing is that the saw cuts into the log becomes oblique and curved due to undesirable geometry. The magnitude of this so-called decomposition error may be 0.2-0.6 mm for the relatively large radius curves that are used today, approximately 40-60 meters. For double blades the situation becomes even worse by saw cuts are broadened and rear-sawing is done. Rear-sawing means that the blade of the circular saw blade's rear part (in the log feed direction) will engage as a result of unwanted side forces that push on the blade, resulting in a wider kerf, or saw cut, in the block than otherwise. This increases chip loss, resulting in a reduced yield.
Furthermore, due to the undesirable side forces that occur on the saw blade, it must be dimensioned to withstand this stress laterally, i.e. to be thicker than otherwise would be necessary. This however increases kerf loss further. In addition, there is also an increased risk of blade deformation or breakage and/or that the saw blade must be replaced more frequently than otherwise which can be costly, not least as the production has to be stopped when the saw is at a standstill due to maintenance.
It may also emerge an increased need to insert a wiper slot in the saw blade, which generates further sawdust and thus lowers the yield further.
A further problem with curve sawing is that the timber often has a stronger curvature than the minimum bending radius possible to comply with a circular saw, resulting in that the grain cannot be followed. Another problem is that the wood may be bent in different directions. In such case, it may not be possible to curve saw the wood so that the grain followed.
It may be concluded that there is a need for improvement to increase yield and reduce kerf loss associated with curve sawing of timber, as well as to increase the quality of sawn timber, especially as economical margins may be strained in the sawmill industry.